The hydrocracking of heavy petroleum fractions is a very important refining process that makes it possible to increase the refinery yield of lower boiling higher value liquid products. For example, heavy feedstocks that cannot be readily upgraded are hydrocracked to lighter fractions such as gasolines, jet fuels and light gas oils. Certain hydrocracking processes make it possible to also obtain a strongly purified residue that can provide good bases for lube oils. Relative to fluid catalytic cracking, an advantage of catalytic hydrocracking is to provide middle distillates, jet fuels and gas oils of very good quality. Conversely, the gasoline that is produced by hydrocracking has a lower octane rating than gasoline that is produced from fluid catalytic cracking.
Hydrocracking is a process that draws its flexibility from several variables, such as the operating conditions used, the type of catalyst used, and the fact that the hydrocracking of hydrocarbon-containing feedstocks can be carried out in one or more stages. One type of conventional hydrocracking catalyst is based on moderately acidic amorphous substrates, such as silica-aluminas. Such systems are used to produce quality middle distillates and optionally lube oil basestocks. These catalysts are used, for example, in two-stage processes.
In processes such as hydrocracking, the step of recycling a hydrogen-rich vapor phase separated from the reaction zone effluent is common. Practical reasons for utilizing this step reside in maintaining both the activity and operational stability of the catalyst used in the process. The recycled hydrogen is typically obtained by cooling the total reaction product effluent to a temperature in the range of about 60° F. (15.6° C.) to about 140° F. (60° C.), and introducing the cooled effluent into a vapor-liquid separation zone. The recovered vapor phase, which contains unreacted hydrogen, is recycled and combined with the hydrocarbon feedstock upstream of the reaction zone.
The art has long recognized the importance of improving the purity (concentration) of hydrogen in the recycle stream of hydroprocessing units, including hydrocracking units. Thus, it has been the goal of the art to provide enhanced efficiencies of hydrogen utilization with little additional energy consumption and without undue deleterious effects on the maintenance or operation of the hydrocracking equipment. It has also been recognized that by increasing the efficient use of hydrogen, existing equipment could be employed to increase the throughput of the feedstock resulting in higher product yields. A further advantage to the more efficient utilization of hydrogen is the reduction in the amount of make-up hydrogen that must be provided by, for example, a hydrogen plant or cryo-unit.
The type of feedstock to be processed, product quality requirements, and the amount of conversion for a specific catalyst cycle life determines the hydrogen partial pressure required for the operation of a hydrocracking unit. The unites operating pressure and the recycle gas purity determine the hydrogen partial pressure of the hydrotreating unit. Since there is limited control over the composition of the flashed gas from the downstream hydrocracker separator, the hydrogen composition of the recycle flash gas limits the hydrogen partial pressure ultimately delivered to the hydrocracking reactor. A relatively lower hydrogen partial pressure in the recycle gas stream effectively lowers the partial pressure of the hydrogen gas input component to the reactor and thereby adversely affects the operating performance with respect to product quantity and quality, catalyst cycle life, etc. To offset this lower performance, the operating pressure of the hydrocracking reactor has to be increased. Conversely, by increasing the efficiency of hydrogen gas recovery and hydrogen concentration, the hydrogen partial pressure of the recycle gas stream is improved. This results in an overall improved performance of the hydrocracking process unit as measured by these parameters.
Various methods have been proposed, some of which have been commercially practiced, that attempt to improve the hydrogen utilization efficiency of the hydrocracking unit by increasing the concentration of the hydrogen in the recycle gas stream. Such processes typically result in significant additional equipment costs and/or require significant changes in operating conditions, such as temperature and pressure, which typically results in increased capital and operating costs.
One process that has been adopted to improve the hydrogen purity of the recycle stream is conventional pressure swing adsorption (PSA). See, for example, U.S. Pat. No. 4,457,384 issued Jul. 3, 1984 to Lummus Crest, Inc. However, in order to incorporate the PSA unit, the pressure of the reactor effluent gas stream must be reduced from about 2,500 psig (175.8 kg/cm2) to about 350 psig (24.6 kg/cm2). Although the purity of the recycle hydrogen stream can be increased to about 99 mol %, the recycled gaseous stream must be subjected to significant recompression to return it to 2,500 psig (175.8 kg/cm2) before introduction into the hydroprocessor feed stream. The net result is that the capital, operating and maintenance costs are substantially increased by the addition of a large compressor that is required when using a conventional PSA unit.
Another method is described in U.S. Pat. No. 4,362,613 to MacLean which uses membranes with pressure drops up to 150 atmospheres and which also incurs substantial capital investment and operating costs.
It is therefore an object of the present invention to provide an improved process for enhancing the efficiency of hydrogen utilization by means that are compatible with existing hydrocracking units. Such means adversely affect the hydrocracker throughput or the overall economies of the system, including capital expenditures and operating expenditures, the latter including maintenance and energy consumption.
As previously noted, the overall operating efficiency of the hydrocracking process unit can be increased if the partial pressure of hydrogen gas in the feed to the reactor can be increased. It is therefore another object of the present invention to improve the operating performance of hydrocracking process units by increasing its throughput capacity.